1. Technical Field
The present disclosure describes methods for separating a sample from a substrate, and particularly relates to a method for separating a small sample region from a substrate such as a semiconductor wafer in an energetic-beam instrument.
2. Background
Certain inspection methods of samples from integrated circuit wafers and other materials require the fabrication of an electron-transparent (<50 nm thickness) area on the sample that contains the region of interest for observation. Typically, a sample is cut out of a semiconductor wafer or other object by use of an energetic-beam instrument such as a focused-ion beam microscope (FIB) for further processing, modification, or analysis; and analyzed or imaged, if desired, using a transmission electron microscope (TEM), scanning electron microscope (SEM), or other means.
Some definitions of terminology in this application follow. The “substrate” need not be a semiconductor device. It may, for example, be a micromechanical device, or any solid substance whatever requiring TEM. SEM, atom-probe, or other analysis, such as particles, granules, biological materials, or thin films. The “energetic-beam instrument” may be either a single-beam FIB, or a multi-beam FIB; the latter having both an ion beam and an electron beam, and possibly also a laser. Typical energetic-beam beam instruments are those manufactured by Carl Zeiss, Inc. of Oberkochen, Germany or FEI Company of Hillsboro, Oreg. An “end-effector” is a component that can manipulate a sample in vacuum when connected to a nano-manipulator; the nano-manipulator being connected to the energetic-beam instrument with vacuum feed-through. The end-effector may be a simply a fine probe or a micro-gripper device. Most illustrations of the disclosed method in this application refer to a probe, but the method disclosed is not limited to cases where a probe is the end-effector. A suitable nano-manipulator is the OMNIPROBE® 200, manufactured by Oxford Instruments, Omniprobe Products, Dallas, Tex. A “cut” with the energetic-beam is a completed cut as here illustrated or described, which “cut” may actually comprise a series of smaller discrete cuts made in the pattern of the completed cut illustrated or described. The terms “first” and “second” as modifiers to distinguish two objects or procedures do not imply a sequence in time unless so stated.
In situ lift-out is a method for performing the entire TEM sample preparation within the vacuum chamber of an energetic-beam instrument. It relies on a manipulator holding an end-effector of some sort. Often, in situ lift-out uses the gas-assisted material deposition capability, or chemical-vapor deposition (CVD) capability of the energetic-beam instrument to connect the excised sample containing the region of interest to the tip of a probe. Alternatively, the sample may be pierced by the probe tip, or clasped by micro-grippers or clamps. The excised sample can then be attached to a TEM grid or other holder by means of gas-assisted material deposition. After the sample has been welded to a probe tip by FIB gas-assisted deposition, the separation of the sample from the probe is made by applying the energetic-beam (generally the ion beam) to sever the connection. In any case, the region of interest can be thinned to an electron-transparent thickness using ion milling. In a previously practiced method for in situ lift-out (U.S. Pat. No. 5,270,552 to Ohnishi, et al.), the tip of the probe is connected to a partially excised sample before the sample is completely released from the wafer. This practice has disadvantages overcome in the present disclosure.
There is always a danger that transient mechanical forces can be transmitted to the sample while it is attached to an end-effector and simultaneously to a second object, such as the substrate or TEM grid. For example, shocks can be initiated by movement of mechanical actuators controlling gas injection, causing relative displacement between the probe and sample, or thermal or mechanical drift of either the end-effector or the stage supporting the sample may occur. Forces arising from the relative displacement caused by these events can change the natural strain state of the material and induce artifacts such as dislocations. In severe instances, fractures and cracks are created.
Prior-art methods share at least one step somewhere in the workflow where a lift-out sample is simultaneously attached to two objects. An example is when an end-effector attaches to a partially excised lift-out sample before its full release from the substrate. Another case occurs after completing the lift-out, where the excised sample is attached to a holder or support while still held by the end-effector.
An important goal of any sample preparation method is the avoidance of creating artifacts in the sample. There is a need for an in situ lift-out method that avoids the danger of damaging a sample or inducing artifacts during lift-out and any following steps, and that also minimizes the time during which a transient mechanical event can place damaging stress on the end-effector and sample or cause unacceptable delays.